#include <pthread.h>
#include <stdlib.h>

#define NHASH 29
#define HASH(id) (((unsigned long)id) % NHASH)

struct foo* fh[NHASH];

pthread_mutex_t hashlock = PTHREAD_MUTEX_INITIALIZER;

struct foo
{
    int f_count;
    pthread_mutex_t f_lock;
    int f_id;
    struct foo* f_next; /* protected by hashlock */
                        /* ... more stuff here ... */
};

struct foo* foo_alloc(int id) /* allocate the object */
{
    struct foo* fp;
    int idx;

    if ((fp = malloc(sizeof(struct foo))) != NULL)
    {
        fp->f_count = 1;
        fp->f_id = id;
        if (pthread_mutex_init(&fp->f_lock, NULL) != 0)
        {
            free(fp);
            return (NULL);
        }
        idx = HASH(id);
        pthread_mutex_lock(&hashlock);
        fp->f_next = fh[idx];
        fh[idx] = fp;
        pthread_mutex_lock(&fp->f_lock);
        pthread_mutex_unlock(&hashlock);
        /* ... continue initialization ... */
        pthread_mutex_unlock(&fp->f_lock);
    }
    return (fp);
}

void foo_hold(struct foo* fp) /* add a reference to the object */
{
    pthread_mutex_lock(&fp->f_lock);
    fp->f_count++;
    pthread_mutex_unlock(&fp->f_lock);
}

struct foo* foo_find(int id) /* find an existing object */
{
    struct foo* fp;

    pthread_mutex_lock(&hashlock);
    for (fp = fh[HASH(id)]; fp != NULL; fp = fp->f_next)
    {
        if (fp->f_id == id)
        {
            foo_hold(fp);
            break;
        }
    }
    pthread_mutex_unlock(&hashlock);
    return (fp);
}

void foo_rele(struct foo* fp) /* release a reference to the object */
{
    struct foo* tfp;
    int idx;

    pthread_mutex_lock(&fp->f_lock);
    if (fp->f_count == 1)
    { /* last reference */
        pthread_mutex_unlock(&fp->f_lock);
        pthread_mutex_lock(&hashlock);
        pthread_mutex_lock(&fp->f_lock);
        /* need to recheck the condition */
        if (fp->f_count != 1)
        {
            fp->f_count--;
            pthread_mutex_unlock(&fp->f_lock);
            pthread_mutex_unlock(&hashlock);
            return;
        }
        /* remove from list */
        idx = HASH(fp->f_id);
        tfp = fh[idx];
        if (tfp == fp)
        {
            fh[idx] = fp->f_next;
        }
        else
        {
            while (tfp->f_next != fp)
                tfp = tfp->f_next;
            tfp->f_next = fp->f_next;
        }
        pthread_mutex_unlock(&hashlock);
        pthread_mutex_unlock(&fp->f_lock);
        pthread_mutex_destroy(&fp->f_lock);
        free(fp);
    }
    else
    {
        fp->f_count--;
        pthread_mutex_unlock(&fp->f_lock);
    }
}
